Influence of corona poling on ZnO properties as n-type layer for optoelectronic devices

Corona poling effects on optical and structural characteristics of zinc oxide (ZnO) thin films prepared by sol–gel spin coating technique were investigated. Atomic force microscope study showed the formation of pyramidal grains structure on the Corona-treated surface. The green–yellow photoluminescence peak centered at 2.36 eV and correlated to the antisite oxygen OZn defect, was found to decrease. X-ray diffraction patterns demonstrated that the Corona treatment enhanced the polycrystalline nature and increased the grain sizes of the ZnO thin films, which was also beneficial for electron transport. The role of the surface roughness of the ZnO thin film as electron transport layer in determining the photovoltaic effect of the inverted solar cells (ISCs) was examined by fabricating ISCs based on P3HT/PC61BM. The power conversion efficiency (PCE) obtained from these fabricated ISCs increased from 3.05 to 3.34%.

Organic solar cells (OSCs) have attracted a lot of interest during the last three decades due to their potential benefits in low-cost solar energy harvesting [1][2][3] . The most common type of OSCs is constructed based on a bulk heterojunction (BHJ) structure, with the photoactive layer composed of a blend of a donor (D)/acceptor sandwiched between a poly (3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS)/indium tin oxide (ITO) anode and a low work function metal top cathode 4,5 . However, achieving high efficiency while maintaining long-term ambient air stability remains a critical problem for BHJ-OSCs. Inverted solar cells (ISCs) are one of the successful approaches for improving the stability and performance of BHJ-OSCs 6,7 . The development of ISCs is entirely dependent on the electrical and surface characteristics of cathode interface layers. As a result, the quest for an electron transport layer (ETL) leads to the use of a large number of metal oxides such as titanium oxide (TiO x ) 8,9 , cesium carbonate (CsCO 3 ) 10,11 , and zinc oxide (ZnO) 12 . Among all these ETL materials, ZnO is used more frequently due to its low work function, which enables the formation of an ohmic contact to be formed with the photoactive layer 13 . As well as ZnO possesses specific characteristics such as low cost, good air stability, nontoxicity, and high transparency in the visible/near-infrared spectral range 14 . There are several deposition techniques used to prepare the ZnO thin films such as atomic layer deposition (ALD) 15 , chemical vapor deposition (CVD) 16 , RF magnetron sputtering 17 , spray pyrolysis 18 , pulsed-laser deposition 19 , electrochemical deposition 20 , and sol-gel spin coating technique [21][22][23] . The sol-gel method offers the possibility of preparing thin film-supported nano-sized particles 24 , excellent control of the stoichiometry, and easy modification of film composition 25 .
In the present study, the influence of Corona poling treatment on the prepared ZnO thin film by the sol-gel method was investigated using atomic force microscope (AFM), UV-Vis absorption, photoluminescence (PL) spectroscopy, and X-ray diffraction (XRD). In addition, the photovoltaic performance of fabricated ISCs using ITO/ZnO/P3HT:PC 61 BM/MoO 3 /Ag architecture was presented. Furthermore, the comparative performance of these devices with and without Corona poling of ZnO thin films has been studied.

Experimental details
Materials. Zinc acetate dihydrate "Zn(CH 3 COO)2·2H 2 O" (99.9% purity), 2-methoxyethanol (99.8% purity), ethanolamine (99.5% purity), and 1,2-Dichlorobenzene (anhydrous, 99%) (DCB) were purchased from Sigma-Aldrich. Poly(3hexylthiophene) (P3HT) with 91-94% regio-regularity, and [6,6]-phenyl C61-butyric acid methyl ester (PC 61 BM) were purchased from Ossila. Indium-tin-oxide (ITO) coated glass substrates with a sheet resistance of (15-20) Ω/sq were obtained from Lumtec, Taiwan. All materials were used as-is, without further purification. www.nature.com/scientificreports/ Preparation of n-type ZnO sol-gel. ZnO thin films were deposited on ITO-coated glass substrates using sol-gel processing method. The ITO substrates were sequentially washed by ultrasonication in a detergent, distilled water, isopropyl alcohol, and acetone for 10 min each. The washed and dried substrates were immediately transferred to oxygen plasma cleaner for 5 min. The zinc precursor was prepared by dissolving 0.5 g zinc acetate dihydrate in 5 ml 2-methoxyethanol with 0.14 mg ethanolamine (as a stabilizer) and stirred for 12 h under ambient air conditions. The thin films were deposited on plasma-washed ITO substrates using the spin-coating technique (3000 rpm, 40 s) 26,27 . The produced films were divided into two comparative films: the first film was annealed at 200 °C for 1 h. For the other film, it was annealed for 1 h at 200 °C under the Corona poling effect (6 kV DC applied voltage, 0.5 cm needle-sample distance) as shown in Fig. 1. After that, the applied voltage was maintained until the film reached room temperature. Both films were transferred into a glove box system for the next deposition steps.

Device fabrication.
A blend of 20 mg P3HT and 20 mg PC 61 BM was dissolved in 1 ml DCB with a weight ratio of 1:1 and stirred at 60 °C for 12 h. Two types of inverted organic solar devices were fabricated: the first device was deposited on the untreated ZnO layer and will be referred to hereafter as D1. The second device was fabricated on top of the Corona-treated ZnO layer and was named D2. A 0.45 µm tetrafluoroethylene (PTFE) filter was used to filter the stirred solution. The filtered solution of P3HT/PC 61 BM was spin-coated on top of both types of ZnO layers at 600 rpm for 1 min and then annealed at 100 °C for 10 min. Finally, MoO 3 (7 nm) and Ag (100 nm) were thermally deposited through a shadow mask forming a device area of 0.06 cm 2 for both devices. MoO 3 essentially acts as a hole transport layer (HTL) and manifestly promotes the ohmic contact between the active layer and Ag anode for accelerating the hole extraction 28 . The inverted structure of both devices D1 and D2 was ITO/ZnO/P3HT:PC 61 BM/MoO 3 /Ag as shown in Fig. 2a.
Characterization. The surface morphology of the ZnO thin films was examined using an atomic force microscope (flex AFM3). Contact mode was used during the scanning with a Nano surf C300 (version 3.5.0.31) software. The UV-visible absorption spectrum of the ZnO thin films was obtained by the JASCO (V-630) UV/ Vis spectrophotometer. Photoluminescence (PL) setup (He-Cd laser, CW, 325 nm, Max.200mW, KIMMON KOHA CO., LTD.) was used for measuring the PL spectra of ZnO thin films. The X-ray diffraction (XRD) patterns have been recorded using a Shimadzu XRD-6000 X-ray diffractometer. The current-voltage measurements have been recorded under 100 mW/cm 2 of AM 1.5 G irradiation using a computer-controlled Keithley 2400 source meter unit.

Results and discussion
Optical properties. Figure 2B represents the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels of the fabricated device. To investigate the influence of Corona poling on the surface morphology of the ZnO film, AFM measurements were performed and are presented in Fig. 3. Upon close examination of the AFM images, it was observed that the surface of the untreated Corona ZnO film consists of elongated grains with a root mean square (RMS) roughness of 28.65 nm (Fig. 3a). However, the ZnO film after Corona treatment shows a pyramidal grains structure and exhibits a large RMS surface roughness of 66.16 nm as shown in Fig. 3b. Therefore, it is apparent that the Corona poling effect can significantly change the morphology of the surface. The increase in the surface roughness may effectively reduce the charge-transport distance and improve the photocurrent Jsc. In addition, the sharp nanoscale texture on the surface of the Corona-treated ZnO thin film may further enhance the electron extraction from the photoactive layer.
The UV-visible absorbance of the ZnO thin films have been investigated in the wavelength range of 300-1100 nm as a function of Corona treatment. As can be seen from Fig. 4a, the wavelength of an excitonic absorption is about 325 nm. The existence of this excitonic peak indicates that the ZnO films have good structural quality. The treated ZnO film by 6 kV shows lower absorption in the visible range (400-700 nm). The value of  www.nature.com/scientificreports/ optical direct-band gap ( E d g ) of the ZnO films was obtained using the following relation by extrapolating the linear component of (αhυ) 2 versus (hυ) plots 29,30 : where α is the absorption coefficient, β is a constant, and hυ is the photon energy. Figure 4b shows the (αhυ) 2 versus (hυ) plots. The value of E d g of the ZnO thin films was observed to decrease from 3.39 to 3.33 eV with the Corona treatment. This decrease in the optical band gap may be ascribed to an increase in grain size.
The photoluminescence (PL) spectra of the prepared ZnO thin films measured at room temperature are plotted in Fig. 5. Two luminescence peaks are observed in both spectra of ZnO thin films, the first peak centered at 3.11 eV near the band edge and is assigned to free exciton emission 31 . The other peak is a broad green-yellow emission located at 2.36 eV, which may be due to the intrinsic defects in the ZnO thin films. There are five types of intrinsic defects in ZnO films; zinc vacancy V Zn , oxygen vacancy V O , interstitial zinc Zn i , interstitial oxygen O i , and antisite oxygen O Zn . Sun used the full-potential linear muffin-tin orbital technique to compute 32 the energy levels of the intrinsic defects in ZnO film as shown in Fig. 6. The energy gap of 2.38 eV from the bottom of the conduction band to the O Zn level matches the energy of the green-yellow emission seen in our spectra. That is, the green-yellow emission was caused mostly by O Zn defects 33 . It is noted that a slight redshift in the UV emission is observed at 3.0 eV for the ZnO treated film by 6 kV. The redshift of the UV emission may be attributed to the increase in grain size. While the PL intensity associated with green-yellow emission has been found to decrease. This decrease in the green-yellow emission may be correlated to the decrease in the concentration of antisite    where λ is the wavelength of the X-ray (λ = 0.154 nm), β is the broadening of the peak at FWHM, and θ is the Bragg angle of the peak. The estimated structural parameters are listed in Table 1. It is observed that the grain sizes increased with the impact of the Corona poling treatment.
Photovoltaic properties. Next, using an ISC device design (see Fig. 2a), we examined the PV characteristics of the fabricated devices D1 and D2 that were deposited on top of the untreated and Corona-treated ZnO layer, respectively. Figure 8 shows representative current density versus voltage (J-V) characteristics of solar cell devices under 100 mW/cm 2 of AM 1.5 G irradiation. The extracted PV data are listed in Table 2. The device D1 exhibits an open-circuit voltage (V oc ) of 0.61 V, a short circuit current density (J sc ) of 9.67 mA/cm 2 , a fill factor (FF) of 51.80%, and power conversion efficiency (PCE) of 3.05%. However, device D2 shows a V oc of 0.61 V, a J sc of 10.46 mA/cm 2 , a FF of 52.38%, and a PCE of 3.34%. It can be noticed that device D2 shows enhanced performance compared to D1. Both J sc and PCE for D2 are enhanced dramatically while the V oc of both devices does not change. The ZnO is widely recognized for its role as a hole blocking and electron-transporting layer. So, the enhanced performance of the device D2 is attributed to the increase in surface roughness of the treated (2) D = 0.9 β cos θ